The present invention relates to hard surfacing techniques generally, and in particular, to deposition techniques applied to extruder screws, such as those used in the screw feed mechanism of polymer injection molding and extrusion operations.
Hard surfacing is the technique of welding or spray coating new or worn parts to combat wear, corrosion, or to build up a part to a larger size, or to repair a machining error or a casting defect. This technique has been used to build up low-carbon steel bearings and shafts for many decades. Although the primary object is to resist abrasion, hard surfacing must also tightly hold onto the base metal so that it will not flake or chip off.
In the polymer products industry, injection molding and extrusion methods are popular ways of making components. A common wear item in both production methods is the xe2x80x9cextruder screwxe2x80x9d, which is used in a screw feed mechanism to deliver polymer pellets or granules which are then heated and passed through a die or into a mold under pressure. Extruder screws are increasingly being exposed to resinous materials having various additives, such as ceramic fillers and glass fibers which abrasively wear nickel-based alloy stainless steel and alloy steel, the popular choices for the base metal of such feed mechanisms. These base metals exhibit high tensile strength, but don""t always hold up very well in high corrosion and wear environments.
Current techniques of achieving greater wear resistance for extruder screws include applying hard surfacing only on the major outer diameter surface of the screw, that portion which is most radially disposed from the central axis of the extruder screw. These hard facing materials have included welding, or spray coating applications, of cobalt or nickel-based alloys containing particles of high hardness, such as tungsten carbide. It has been determined that tungsten carbide particles have a tendency to wear other steel and stainless steel parts contacting these alloys in service. Tungsten is also very expensive and difficult to obtain on a regular basis.
Alternatively, ceramic materials have been applied to provide abrasion resistance to the flight surfaces of conveyor screws. In one commercial application, curved flight liners made of alumina ceramic tiles are fastened to the screw flight. The flight tile manufactured by Norten Pakco Industrial Ceramics division of Saint-Gobain Corporation, Latrobe, Pa., under the brand name Wearpak A96, can be designed to match the contour of the screw. These tiles are installed using high strength adhesives and welded inserts. While alumina oxide ceramic is one of the hardest materials known, next to a diamond, adhering tiles along the leading edge of a conveyor screw is expensive and time consuming. Such a process also does not provide a smooth flight surface, which can bind plastic particles and inadvertently increasing frictional contact.
Other methods of employing ceramic materials have also been used to protect extruder screws for injection molding machines or extrusion machines. For example, in U. S. Pat. No. 4,729,789, issued Mar. 8, 1988, a process for providing a sintered layer on a metallic core is provided whereby the sintered material creates a series of flights. The ceramic material is initially applied to a steel bar as a xe2x80x9cgreenxe2x80x9d compact, followed by a molding and isostatic pressing operation at a temperature at which the green compact is sintered to result in a xe2x80x9cnear net shapexe2x80x9d extruder screw. While such a method does produce a ceramic material having a steel core, the resulting product is difficult to repair in service, since it would require repairing a structural ceramic material, rather than merely re-hard surfacing a steel flight. It is also unclear as to how such ceramic flights will resist cyclical roads having high tensile forces, since ceramic materials, by themselves, are known to be fairly brittle in tensile or torsional loading situations.
Accordingly there remains a need for a new or repaired extruder screw having high wear protection especially in the minor outer diameter flight sides or the root areas of an extruder screw. Such a need is especially apparent in the repair industry in which current industrial trends require perpetual rebuilding capability in order to reduce inventory and minimize down time.
In a first embodiment of the present invention, a wear resistant extruder screw is provided which is capable of being used in material conveyance, such as in transporting resin in injection molding and extrusion operations. The wear resistant extruder screw includes a base alloy extruder screw body having a plurality of flights, a major outer diameter and a minor inner diameter. Each of the flights of the extruder screw has a pair of flight sides disposed between the major outer diameter and the minor inner diameter. The flight sides and the minor inner diameter define a root area proximate to their juncture. In an important aspect of the first embodiment of this invention, a bond coating layer having a thickness of about 1-10 mils is disposed over a major portion of the extruder screw body which includes the root area. Disposed over the bond coating layer is a wear resistant coating layer having a thickness of about 10-100 mils, and an ASTM G65 dry abrasion wear rating of less than 10 mm3/kilometer.
The extruder screws of this invention, are not limited to extrusion operations, as their name suggests, but can be employed in resin or material conveyance and injection molding operations as well. Such operations can include the transportation of grain, chemicals, waste, animal products, minerals (including augers), coal, wood fiber, and all sorts of industrial materials. They can be designed in flight sizes ranging from inches to feet, and provide wear resistance in the minor inner diameter, flight sides and root area of the extruder screw, where wear has been known to reduce screw life.
After the preferred wear resistant and bond coating layers wear during service, they can be perpetually replaced with further coating operations on site, or within a short distance, to a thermal spraying repair shop, without waiting for a new extruder screw to be manufactured. While prior art ceramic only and ceramic tile systems provide some measure of protection, they are either ignoring the vital root area of the screw, or contain elaborate and expensive sintering operations which do not lend themselves to on-site repair.
In further embodiments of this invention, the base alloy extruder screw body is prepared with an undercoating layer having a thickness greater than about 1 mil disposed over at least the root area. The undercoating layer desirably has a coefficient of thermal expansion which is (1) less than the coefficient of thermal expansion of the base extruder screw body, (2) greater than that for the wear resistant coating, or (3) both. Disposed over the undercoating layer of this embodiment is a wear resistant coating layer having a thickness of at least about 10 mils.
It will be understood by the teachings of this invention that the undercoating layer desirably bridges the thermal expansion capabilities of the base alloy extruder screw body and the wear resistant coating. Most desirably, the undercoating or bond coating layer of this invention has a coefficient of thermal expansion which is in between that of the wear resistant coating and the base alloy extruder screw body. In more preferred embodiments of this invention, a wear resistant coating layer is applied by thermally spraying molten ceramic material obtained from a rod or pellet form, and constitutes the application of substantially molten ceramic particles, as opposed to semi-molten particles which are known to form voids and inconsistencies in coatings bonded to base alloy screw bodies.